In the United States, thyroid abnormalities are common endocrine disorders for which thyroid hormones are frequently prescribed. Levothyroxine (L-T4) is often considered the thyroid hormone of choice due to its long half-life, stability, uniform potency, lack of allergenic foreign protein, and relative low cost. Thyroid function tests (TFTs, free and total thyroxine, total triiodothyronine, and thyrotropin) are used to monitor efficacy of L-T4 therapy. Studies have suggested that L-T4 has a 9-hour distribution phase, resulting in a 9 to 55 percent increase in free and total thyroxine and a 23 to 40 percent decrease in thyrotropin serum concentrations after L-T4 administration. Although these studies have suggested monitoring therapy based on blood specimens collected before the usual daily L-T4 administration, the state-of- the-art practice is to assess TFTs after L-T4 administration. Further, it is unclear whether the changes in thyroid stimulating hormone (TSH) are due to drug administration or related to normal diurnal variation. This randomized, prospective, open-label, cross-over study, involving 44 patients receiving L-T4 therapy will evaluate the effects of L-T4 administration time on TFTs. Patients will be randomly assigned to either Group A (seven suppression, seven replacement, and eight high-dose patients who will receive their usual L-T4 dose after serial blood sampling to determine TFTs) or Group B (seven suppression, seven replacement, and eight high-dose patients who will receive their usual L-T4 dose after serial blood sampling for determination of TFTs). After a washout period of not less than l week, patients will be crossed over to the alternate group. Pharmacokinetic parameters (Cmax, Tmax, Kel.TI/2, AUC, Vd, F) will be compared statistically between groups. This study is inactive. Although results have not been formally analyzed, it appears thyrotropin serum concentrations follow a diurnal pattern in both treatment groups.